1. Field of the Invention
The present invention relates to interconnection layers for solid oxide electrolyte, electrochemical cells.
2. Description of the Prior Art
High temperature electrochemical cells are taught by Isenberg, in U.S. Pat. No. 4,490,444. In these type of cells, typified by fuel cells, a porous support tube of calcia stabilized zirconia, has an air electrode cathode deposited on it. The air electrode may be made of, for example, doped oxides of the perovskite family, such as LaMnO.sub.3. Preferred dopants are Sr, Ca, Co, Ni, Fe, and Sn. Surrounding the major portion of the outer periphery of the air electrode is a layer of gas-tight solid electrolyte, usually yttria stabilized zirconia. A selected radial segment of the air electrode is covered by an interconnection material. The interconnection material may be made of a doped lanthanum chromite film. Suggested dopants are Mg, Ca, and Sr.
Both the electrolyte and interconnect material are applied on top of the air electrode by a modified chemical vapor deposition process, with the suggested use of vaporized halides of zirconium and yttrium for the electrolyte, or vaporized halides of lanthanum, chromium, magnesium, calcium or strontium for the interconnection. material, at temperatures of up to 1450.degree. C., as taught by Isenberg, in U.S. Pat. No. 4,597,170, and Isenberg et al., in U.S. Pat. No. 4,609,562.
It has been found that there are certain thermodynamic and kinetic limitations in doping the interconnection from a vapor phase by a chemical vapor deposition process at 1300.degree. C. to 1450.degree. C. The vapor pressures of the calcium chloride, strontium chloride, cobalt chloride, and barium chloride are low at vapor deposition temperatures, and so, are not easily transported to the reaction zone at the surface of the air electrode. Thus, magnesium is the primary dopant used for the interconnection material. However, magnesium doped lanthanum chromite, for example La.sub.O.97 Mg.sub.0.03 CrO.sub.3, has a 12% to 14% thermal expansion mismatch with the air electrode and electrolyte materials. Additionally, halide vapors at 1300.degree. C. to 1450.degree. C. can interact with the air electrode material during the initial period of interconnection application. This causes, in some instances, air electrode leaching of main constituents, such as manganese, into the interconnection material providing a Mn-Cr rich interconnection phase at the interconnection-air electrode interface. During prolonged cell operation, the Mn can diffuse into the interconnection bulk and cause possible destabilization effects.
In an attempt to solve some of these problems, Isenberg et al., in U.S. Pat. No. 4,598,467, suggested applying a separate, vapor deposited, interlayer of, for example, calcium and cobalt doped yttrium chromite, about 1 micron thick (0.001 millimeter), between the air electrode, and the interconnection material and electrolyte. This, however, added another step to the process, adding further expense and complication. Additional potential problems with the vapor deposited interconnection material are a certain amount of interface porosity, non-uniform distribution of the Mg dopant, leading to decreased conductivity, and a possible minor amount of gas leakage resulting from non-uniform densification.
Ruka, in U.S. Pat. No. 4,631,238, in an attempt to solve interconnection thermal expansion mismatch problems, taught cobalt doped lanthanum chromite, preferably also doped with magnesium, for example LaCr.sub.0.93 Mg.sub.0.03 Co.sub.0.04 O.sub.3, as a vapor deposited interconnection material, using chloride vapors of lanthanum, chromium, magnesium, and cobalt.
None of these solutions, however, solve all the potential problems of thermal expansion mismatch, Mn leaching from the air electrode, concentration of Mg dopant near the air electrode interface, and interface porosity, and the limitations of doping calcium, strontium, cobalt, and barium by vapor deposition, in a simple and economical fashion. It is an object of this invention to solve such problems.